A superconducting quantum interference device (SQUID) is used to measure very fine magnetic signals of several tens to several hundreds of femtotesla (fT) which are generated by human activities of brain, heart, muscles, and the like. In particular, the SQUID may be medically used in brain function mapping, disease diagnosis of localization of an epilepsy occurrence position, and cognitive function diagnosis by analyzing magnetic signals generated from the brain. However, the SQUID should be cooled to −265 degrees Celsius to operate a SQUID sensor.
In a cooling method that is currently used commercially, a sensor is directly cooled using liquid helium. However, the maintenance cost of a magnetoencephalography (MEG) apparatus is rapidly increasing with supply and demand instability of helium gas which results from global exhaustion of fossil resources. In addition, a user and a subject may be exposed to a very dangerous environment when they treat a cryogenic coolant of −270 degrees Celsius. To overcome these disadvantages, a SQUID apparatus for measuring a biomagnetism using a cryocooling technique has been actively developed.
Coolers have various cooling types such as pulse tube type, Gilfford-Macmahon type, and Joule-Thompson type. Materials used in a regenerator of a cryocooler are Er3Ni, Pb, and the like, which generate a cyclic magnetic noise according to a high-pressure pulse as they are magnetized by their phase changes depending on temperature change. The generated cyclic magnetic noise has several hundreds of micro Tesla and is several to hundreds of thousands times greater than a biomagnetic signal desired to be measured. The magnetic noise distorts a signal to have a great influence on signal analysis.
An indirect cooling technique uses a metal having a high thermal conductivity to effectively transfer heat from a cryocooler to a SQUID sensor. Thus, a thermal noise generated from the metal may significantly reduce operation stability and sensitivity of a SQUID apparatus.
In an existing developed cryocooled SQUID apparatus, a SQUID sensor and a cold head of a cryocooler are integrated into one body. After a cyclic magnetic noise generated from the cold head is removed using a first-order gradiometer or a second-order gradiometer, and a remaining magnetic noise is removed using a digital filter. A signal processed with the digital filter may cause information distortion or loss. Moreover, since a thermal noise generated from a metal has no constant cyclicity, the thermal noise cannot be removed using the digital filter. Thus, sensitivity and stability of the SQUID apparatus are restricted.
After a typical SQUID apparatus using liquid helium is cooled using a coolant, it is very difficult to change a shape or a position of the typical SQUID apparatus. Thus, cognitive function measurement according to a subject's posture and measurement for disease diagnosis cannot be performed using only one apparatus. When a cryocooling technique is introduced to address this shortcoming, structural change is required.
A supersensitive SQUID sensor has been used in various applications such as medical, defense, resource exploration, and space exploration. In particular, the supersensitive SQUID sensor has been actively used in medical fields such as disease diagnosis and brain activity research by measuring a fine magnetism generated from heart and brain. However, high-priced liquid helium should be used to operate a low-temperature SQUID sensor and a special magnetically shielded room having a high magnetic shielding rate is required to reduce an environmental noise and an earth magnetic field that is millions of times greater than an MEG signal. These two problems act as great limitation when a SQUID system is used in a medical field.
Accordingly, various techniques have been developed to reduce consumption of liquid helium. For example, using a closed cycle cryocooler, a helium (He) circulation system is developed, a thermal shield of a low-temperature coolant storage container is cooled or a SQUID sensor is directly cooled.
Daikin Corporation directly cooled a SQUID system using a GM-type cryocooler, instead of liquid helium, to measure a magnetocardiography (MCG) signal and a magnetoencephalography (MEG) signal. However, many added averages and digital signal processes should be used due to an influence of a magnetic noise caused by a cycle noise of the cryocooler. As a result, disadvantages such as signal loss and distortion occur.